Load impedance responsive feedback for variable reactance transformer



LOAD vLMPEDANCE RESPONSIVE FEEDBACK FOR l1545151731970' MLBWJCKl3,505,588

VARIABLE REACTANGE TRANSFORMER Filed March 27, 1968 United States PatentO 3,505,588 LOAD IMPEDANCE RESPONSIVE FEEDBACK FOR VARIABLE REACTANCETRANSFORMER Elwood M. Brock, R.D. 5, Quakertown Road, Flemington, NJ.08822 Filed Mar. 27, 1968, Ser. No. 716,395 Int. Cl. G05f 1/14; H02j3/10 U.S. Cl. 323-6 8 Claims ABSTRACT OF THE DISCLOSURE FIELD OF THEINVENTION The present invention relates, in general, to variablereactance transformers of the type set forth in my prior Patent No.3,343,074 which issued on Sept. 19, 1967, and more particularly toimprovements therein which permit automatic control of the outputcurrent in responseto changes in the load impedance.

DESCRIPTION OF THE PRIOR ART Electric furnaces have, in recent years,Ibecome incrrasingly important in the fields of metallurgy, glassmanufacture and in other related areas, and are often used inhigh-quality manufacturing processes wherein careful regulation oftemperature is required. These furnaces require extremely high currentsto produce the temperature required in the processes for which they aredesigned, the current ow through an electric resistance furnace oftenreaching 1,000 amperes or more. However, said furnaces present a seriousproblem of control, for when an electric furnace is cold, the resistanceelements have a low impedance and the application of power can result ina very high and destructive current flow. To prevent circuit breakers inthe current supply from opening, with the resultant shut-down of thefurnace, this high burst of current must be controlled in some manner.

Early attempts at controlling the cold resistance current flow involvedthe use of mechanically variable load resistors to gradually introducecurrent to the furnace. This type of control is unsatisfactory for manyreasons, and more recently silicon controlled rectifiers have been usedfor this purpose. However, silicon controlled rectiiiers have thedisadvantage that when they become conductive they become fullyconductive and thus permit very short, but very high current bursts tobe applied to the furnace resistance elements, These high current burstsprovide great mechanical strain on the system and do not really solvethe problem of excessive current flow. Further, when a fault condition,such as a short circuit, develops in the furnace elements, neither thesilicon controlled rectifiers nor the mechanically variable loadresistors are able to provide the necessary control, and the circuitbreakers are opened. However, the high currents which flow before thebreakers can open often cause considerable damage to the system and tothe control circuitry, in particular.

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SUMMARY oF THE INVENTION It is an object of the present invention toovercome the disadvantages of the prior art and to provide a variabletransformer which will control the power applied to a variable load inaccordance with the impedance of that load and will transform voltageand current, both functions being performed in a single unitarystructure.

A further object of the invention is to provide a variable reactancetransformer for controlling the current to a resistance furnace load inaccordance with the impedance of that load.

An additional object of the invention is to provide a variable reactancetransformer having both a feedback control and an external control,whereby the system is sensitive to load impedance, and wherein controlcan be obtained through a smaller and less expensive external controlunit than was possible with prior devices.

It is a further object of the invention to provide a variable reactancetransformer control for a variable impedance load which will respond tofault conditions in the load to prevent circuit damage.

A further object of the invention is to provide a modifeld variablereactance transformer which has particular utility in the control ofelectric furnaces by providing a feedback arrangement which givesautomatic control in response to changes in the load impedance. Thisinvention permits an electric furnace to be turned on and graduallybrought up to the desired temperature without the shock of high currentbursts that are produced in presently used silicon controlled rectifierunits. The transformer is comprised of a main annular core and twoauxiliary, or control, annular cores which carry toroidal windings. Themain core carries the secondary, or load, windings of the transformer aswell as the feedback supply windings. The auxiliary cores each carrybalanced but series opposed external control windings operated from anexternal control source such as a magnetic amplifier. The latter coresalso carry balanced, series opposed feedback control windings suppliedfrom the feedback supply windings. The primary input windings surroundthe cores, each turn of the primary encompassing all three cores in atoroidal winding. In a preferred construction, the secondary, feedbackand control windings are adjacent their respective cores, with theprimary being wound outside these windings.

Although the variable reactance transformer is suitable for use with anyvariable load, its unique operating features make it particularlyvaluable with a load such as an electric resistance furnace. When theload impedance presented by a furnace is at a low value, as when thefurnace elements are cold, there tends to be a high current in thesecondary winding of the transformer. The low load irnpedance means thatmost of the flux in the main core will go to producing load current;thus, only a small feedback current will be induced. This low feedbackcurrent, in turn, varies the saturation level of the control cores in adirection to increase their reactance and thus to increase the impedanceof the variable reactance transformer. The increased impedance of thetransformer reduces the output load current. As the electric furnaceresistance elements heat up, their impedance increases, tending todecrease the load current. This permits a larger feedback current to beinduced, increasing the saturation of the auxiliary cores and reducingtheir reactance. This reduced impedance of the transformer permits theload current gradually to increase until the system reaches full loadpower when the furnace resistance element is at the proper temperature.Thus, the transformer senses and responds to the load impedance.

This response to load impedance is particularly useful when a shortcircuit develops in the furnace elements. Such a condition produces highcurrents in both the furnace and the control circuitry which, in priorsystems, could cause considerable damage before the circuit breakerscould respond to cut off the current supply. In the present system, onthe other hand, the start of such high fault currents immediatelyincreases the impedance of the variable reactance transformer andreduces its output. The operation of the present system has beendemonstrated by dropping a buss-bar across the output terminals of thevariable reactance transformer when it is operating at full power toproduce a sudden and complete short circuit condition. The currentoutput immediately dropped to a low level, the change in load currentlevel occurring sufciently quickly to prevent damage to the system andto prevent the circuit breakers from opening. No other known system iscapable of reacting in this manner. Although some transient peaks werepresent immediately upon occurrence of the short circuit, the responseof the variable reactance transformer of the present invention wassuiiciently fast to limit both their amplitude and duration enough toprevent damage to the system.

BRIEF DESCRIPTION OF THE DRAWINGS The novel features which are acharacteristic of the invention are set forth with particularity in theappended claims, but the invention and its objects will be understoodmore clearly and fully from the following detailed description taken inconjunction with the accompanying drawings in which the single figure isa schematic diagram of a variable reactance transformer constructed inaccordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to the schematicdiagram of the single figure, there is illustrated a variable reactancetransformer circuit connected in accordance with the principles of thepresent invention. The transformer is comprised of a main core 12 andtwo auxiliary cores 14 and 16. These cores are annular in shape and arecomposed of any suitable magnetic material. They are so proportionedthat the auxiliary cores 14 and 16 may become saturated when under theinfluence of a normal range of external control and feedback controlcurrent. Neither the auxiliary cores nor the main core necessarilyexhibit the so-called square` loop hysteresis curves, but preferably areof conventional magnetic materials.

Auxiliary cores 14 and 16 carry external control windings 18 and 20,respectively, connected in series opposition through a variable controlresistor 22 to the output of an external control source such as magneticamplier 24. The magnetic amplifier may be of conventional construction,providing an output current having a maximum value of 2 amperes. Theoutput of the magnetic amplifier is controlled by means of a variable DCsource 26 and is supplied with AC power from a suitable AC source 28.

A secondary winding 32 is wound on the main core 12 and supplies thecontrolled and transformed output current to a load device 35 connectedbetween terminals 34 and 36. As has been noted, the load device may bethe resistance elements are diagrammatically indicated in phantom at 37and of an electric furnace, which elements may, forexample, exhibit aresistance of 0.03 ohm at maximum temperature. The secondary winding 32is coupled to the main core 12 only, for isolation from the DC controlcurrents.

Surrounding all three of the cores 12, 14 and 16 is the unitary primarywinding 40 which is connected at ter-I minals 42 and 44 to a source ofalternating current. As diagrammatically illustrated, the turns ofwindings 40 are not separately wound on each of the three cores, buteach turn of the primary winding passesaround all three cores. Thisprimary winding normally carries the input current to the device, whichcurrent is to be transformed and/ or controlled. Where a furnaceresistance element is to be supplied, the power source to which theprimary winding is connected may be a 480 volt AC line, with the primarywinding carrying a full load current of approximately 63 amperes.

The transformer windings so far described are those which are thewindings illustrated and discussed in my above-mentioned Patent No.3,343,074. The improvement represented by the present invention isillustrated by feedback winding 50 which is located 0n main core 12 andthus is responsive to the changing flux induced within that core. Theoutput of feedback supply winding 50 is applied across a potentiometer52 having `a slidable arm 54 which permits the feedback current to bevaried. This variable output is applied across the primary winding of afeedback transformer 56, the primary being connected between slidablearm 54 and one end of potentiometer 52. The secondary winding oftransformer 56 is fed through a fullwave rectifier comprised ofrectiiiers 58 and 60 having their cathodes connected to respective endsof the secondary winding and their anodes connected in common to a DCfeedback line 62. The secondary winding of transformer 56 has a centertap to which is connected the second line 64 of the DC feedback circuit.Connected in series across lines 62 and 64 are the feedback controlwindings 66 and 68 which are wound on auxiliary cores 14 and 16,respectively. Feedback control windings 66 and 68 are connected inseries opposition to each other, but are wound on their respective coresin such a direction as to be additive to the effect of the externalcontrol windings 18 and 20 also wound on the respective auxiliary cores.The circuit relationship of the various control windings is illustratedin the drawing in conventional manner by polarity dots, and since thesewindings are polarity sensitive, they must be connected as illustratedto obtain the desired control results. The two auxiliary cores aredriven along their saturation curves in opposite directions by reason ofthe stated connection of the windings, and thus each core is adapted tocontrol one half-cycle of the applied current, with the two auxiliarycores working together to provide balanced full wave control.

The physical relationship between the several cores and between thecores and their respective windings has not been illustrated herein,since such realtionships are fully illustrated and described in my priorPatent No. 3,343,074. Thus, in a preferred embodiment, the main core 12would be sandwiched between the auxiliary cores 14 and 16, the coresbeing as closely adjacent one another as the bulk of the windings willpermit. The cores are annular and axially aligned, with the controlwindings 18, 20, 66 and 68 being wound on the auxiliary cores only, thesecondary winding 32 and feedback supply winding 50 being wound on themain core 12 only, and primary winding 40 being wound on all threecores. Although my prior patent does not show the feedback controlwindings 66 and 68, it should be understood that these windings aretoroidally wound on their respective cores either adjacent to thecontrol windings 18 and 20, whereby each winding surrounds a portion ofits associated core, or, where both control windings are woundconcurrently, with alternating turns around the full circumference ofthe core. Another possibility would be to arrange the control windingsconcentrically, so that the external control and the feedback controlwindings on each core would form toroids around their respective cores,one winding surrounding the other.

As illustrated in my prior patent, secondary winding 32 is toroidallywound on the main core 12 fo-r isolation from the direct current controlflux induced in the auxiliary cores. Again, the feedback supply winding50 is not illustrated in my prior patent, but it will be understood thatthis winding may be wound on the main core adjacent to the secondarywinding 32, with each winding thus taking up a portion of thecircumference of the main core. Alternatively, the two windings may bewound together with alternating turns, or they may be Woundconcentrically; in either case, both windings would extend around thefull circumference of the core. The primary winding 40 is inductivelycoupled to all three cores, with each turn of this winding passingaround, or surrounding, all three cores and their respective secondary,feedback and control windings.

The feedback supply winding 50, which may provide an output of up to 40amperes in one embodiment of the invention, is normally designed with asufficient number of turns so that its output can be adjusted by meansof potentiometer 52 to supply approximately 85 percent of the controlpower required by the system. When properly arranged, the controlwindings 18 and 20 will provide the remaining 15 percent of the controlpower, permitting a substantial reduction in the capacity and size ofthe usual external control source. With this proportion, the `externalcontrol source will then provide a Vernier control of the transformerwith the main controlling effect being derived from the feedbackcircuit. It has been found that about 85 percent is the maximum amountof control power that should be provided by the feedback windings. lfadditional feedback control is provided, the external control will nothave much effect upon the operation of the system. In such acircumstance, the system would go to full power and remain there; theexternal control source is ineffective to decrease the power output ofthe variable reactance transformer. However, at 85 percent the externalcontrol power source is capable of effectively regulating the system.

The operation of the basic variable reactance transformer is set forthin some detail in my prior patent, above-mentioned, which describes themanner in which the degree of saturation of the auxiliary cores affectstheoutput appearing on the secondary winding 32. As noted in thatpatent, lby making the DC external control source variable, any desireddegree of saturation may be attained in cores 14 and 16 and varyingamounts of energy will thus be available to drive the main core 12 andthe secondary Winding 32. Whereas the entire control power in the priordevice was obtained from the external control source, the presentinvention is distinct in that it is made responsive to the loadimpedance through the use of the described feedback circuit. Thus, whenthe load impedance is at a low value, there tends to be a high currentin the output winding 32 of the variable reactance transformer and,simultaneously, a low feedback current in feedback supply winding 50.The feedback current induced in winding 50 is converted to a directcurrent and is applied to the control cores 14 and 16, decreasing thesaturation of the control cores and increasing the impedance of thevariable reactance transformer, thus reducing the output load current.As the impedance of the load increases, the feedback current induced inwinding 50 increases, causing the impedance of the auxiliary cores todecrease. This decrease in transformer impedance permits the loadcurrent to increase to compensate for the increase in load. As the loadimpedance continues to go up, the feedback current continues toincrease, reducing further the impedance of the auxiliary cores andpermitting the load current gradually to increase until the systemreaches full power when the load impedance is at the design level. Thus,the variable :reactance transformer senses and responds to the loadimpedance to control the load current and prevent that current fromexceeding the desired level.

In the particular embodiment of the invention which has been describedherein, with an electric resistance furnace as the load for the variablereactance transformer, which load would be approximately 0.03 ohm atmaximum temperautre, and with the furnace at its maximum temperature,the system may be adjusted by setting the potentiometer 52 to provideapproximately 50 percent of its available feedback current. The externalcontrol source 24 is then set to its maximum control level, and thepotentiometer 52 is adjusted to obtain the maximum design output fromthe secondary winding 32. Under this operating condition, reduction ofthe external control current should decrease control core saturationenough to cause a proportionate decrease in the output current in thesecondary winding 32. If too much feedback current is present, thefeedback winding will overpower the control circuit and reduction of theoutput from magnetic amplier 24 will not affect the output appearingacross terminals .34 and 36. As has been noted, the normal proportioningof control would nd the feedback control winding supplying percent ofthe control energy with 15 percent being supplied from the magneticamplitier. However, the exact proportion of feedback to external controlis a function of the exact load impedance.

Once the system has been properly adjusted for its load, subsequentapplications of the variable impedance load will be handledautomatically by the system, with the initial current to a low impedancebeing held to a relatively low value and gradually increasing to thedesiredlevel as the impedance increase to its design value. Thisoperation is fully automatic and is a distinct improvement over priorart methods of controlling such loads. Although the system has beendescribed in, and is particularly useful with, an electric resistancefurnace, and specific current and load resistance values have beengiven, it will be apparent that other variable impedance loads may 4beused with this system and the load current similarly regulated at theselected levels.

Not only does the variable reactance transformer permit a gradualincrease in load current to protect both the load and the power inputsource, but the transformer system also acts to protect itself againstfault conditions f in the load. -As has been explained, the feedbackarrangement of the present invention is adapted to respond to a faultcondition such as a short circuit sulicicntly quickly to prevent damageto the system. For example, if the normal full load secondary current is1,000 amperes, it would be expected that a short circuit in the loadwould produce a much higher current condition. However, the tendencytoward a high current condition caused by a short circuit immediatelydecreases the feedback current and thus increases the impedance of thetransformer. It has lbeen found that with a transformer of the presenttype, the maximum available secondary current under short circuitconditions would be approximately 500 amperes with the full two amperecontrol current from magnetic amplifier 24. With the direct current fromthe magnetic amplier being reduced to zero, the short circuit currentthrough the load would be less than 400 amperes. Since the system isdesigned to handle a secondary current of 1,000 amperes, it will causeno damage to the system.

Thus, there has been described a variable reactance transformer havingan improved response time, power factor and eciency which, by reason ofthe use of feedback control circuitry, is responsive to load impedanceand permits a simplied, and thus less expensive, low power externalcontrol source. Many Variations and modications of the inventive conceptherein described will tbe apparent to those skilled in the art, and itis therefore desired that the foregoing description be taken asillustrative.

I claim:

1. In a variable reactance .transformer having a main core and a pair ofauxiliary cores located adjacent thereto, a secondary winding wound onsaid main core only, an external control winding wound on each of saidauxiliary cores, said external control windings being connected inseries opposition, and a primary winding surrounding said main andauxiliary cores, the improvement comprising: a variable impedance loadconnected across said secondary winding, feedback circuit meansresponsive to the impedance of said load to control the current suppliedto said load by said secondary winding, said feedback 7 circuit meansincluding a feedback supply winding wound on said main core, convertermeans for converting alternating feedback current to direct feedbackcurrent, and a feedback control winding lwound on each of said auxiliarycores, said feedback control windings being connected to said convertermeans in series opposition and being wound on said auxiliary cores toaid the corresponding external control windings, whereby a reduced loadimpedance produces a reduced feedback current and an increased reactanceof said variable reactance transformer to reduce said current suppliedto said load.

2. The variable reactance transformer of claim 1,

wherein said feedback circuit further includes lvariable resistance.means connected to said feedback supply winding to permit adjustment ofsaid feedback current.

3. The variable reactance transformer of claim 2, wherein said convertermeans includes a feedback transformer and rectifier means for convertingalternating feedback eurent from said feedback supply winding to directfeedback current.

4. The variable reactance transformer of claim 1, further including asource of external control current for said external control windings,said feedback circuit means providing approximately 85 percent and saidsource of external control current supplying the remainder of the totalcontrol current required by said variable reactance transformer.

5. The variable reactance transformer of claim 3, wherein said source ofexternal control current is variable to regulate the reactance of saidauxiliary cores and thus to provide a Vernier control of the currentsupplied to said load.

6. In a variable reactance transformer having control means responsiveto load impedance, a main core and rst and second auxiliary controlcores; a secondary Winding wound on said main core; variable impedanceload means connected across said secondary winding; a feedback supplywinding wound on said main core; first and v second external controlwindings and rst and second feedback control windingswound on said firstand second auxiliary cores, respectively; an external 4variable sourceof DC control current, said external control windings lbeing connectedin series opposition across `said external source of control current;rectifier` means; said rst and second feedback control windings beingconnected in series opposition through said rectier means to saidfeedback supply winding; a source of power; and a, primary windingconnected across said source of power and wound on said main core andsaid first and second auxiliary cores, whereby the feedback currentinduced in said feedback supply winding is proportional to the impedanceof said load, the feedback current and the external control currentserving to vary the reactance of said rst and second auxiliary cores toregulate the power supplied through said variable reactance transformerto said load.

7. The variable reactance transformer of claim 6, further includingvariable impedance means in circuit with said feedback supply winding topermit adjustment of the feedback current level, whereby said feedbackcurrent provides approximately and said external control currentprovides approximately 15 percent of the total control energy suppliedto said lirst and second auxiliary cores.

8. The variable reactance transformer of claim 6, wherein said loadmeans is an electric furnace resistance element the impedance of whichvaries with temperature.

References Cited UNITED STATES PATENTS 2,586,657 2/1952 Holt 323-56 X2,870,397 l/l959 Kelley 323-56 3,123,764 3/1964 Patton 323-56 LEE T.HIX, Primary Examiner G. GOLDBERG, Assistant Examiner U.S. Cl. X.R.

